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COBOL for the 21st Century

Nancy Stern Hofstra University

Robert A. Stern Nassau Community College

James P. Ley University of Wisconsin-Stout (Emeritus)

John Wiley & Sons, Inc.11th edition

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Iteration: Beyond the Basic PERFORM

Chapter 9

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In-Line PERFORM

PERFORM...

END-PERFORM• Use when only a few statements are to

be executed• Modularize with PERFORM paragraph-

name when many statements required

Statements to be executed

Format

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Nested PERFORM

• PERFORM may be one of instructions in range of another PERFORM

Perform 200-Paragraph .

.

200-Paragraph.

Perform 500-ParagraphNested PERFORM

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Nested In-Line PERFORM

• In-line PERFORMs can include nested in-line PERFORMs or PERFORMs with paragraph-name

Perform. . .

Perform . . .

End-Perform. . .

End-Perform

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PERFORM UNTIL

PERFORM [paragraph-name-1]

UNTIL condition-1

• Repeats statements in paragraph until condition is true

• Called iteration or loop

Format

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Loop Example

• Write Hello-Record-Out 3 times

Move Zeros To Count

Perform Until Count = 3

Write Hello-Record-Out

Add 1 To Count

End-Perform

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PERFORM … TIMES

• Executes a sequence of steps a fixed number of times

• No counter needed

• Loop below executes paragraph 300-Print-Rtn 5 times

Perform 300-Print-Rtn

5 Times

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Loop Example

Sum even integers from 2 through 10

• Initialize a field to first number to be added (2)

• Increment field by 2 so it equals even numbers (2, 4, 6, 8, 10)

• Use this field's value to– Test in condition– Add to a total field to find sum

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Code for Loop Example

• Sum even integers from 2 through 10

Move 0 To Total

Move 2 To Count

Perform Until Count > 10

Add Count To Total

Add 2 To Count

End-Perform

Display 'Total=', Total

Initialize field to be tested

Test field until it reaches desired value

Change field tested so condition eventually met

Result: Total = 30

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Nested PERFORMs

• One of statements in PERFORM loop may be another PERFORM loop

• A loop within another loop is called a nested loop

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Nested PERFORM Pseudocode

Perform 5 TimesPerform 10 Times

Read record from file and add its amount to group total

End-ReadEnd-PerformPerform Print-Group-Total

End-Perform

Outer loop

Inner loop

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TIMES vs UNTIL

• Use PERFORM … TIMES if you know in advance the number of times loop statements are to be executed

• Use PERFORM … UNTIL if number of times loop repeated is needed for output or calculations

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PERFORM VARYING

PERFORM VARYING identifier-1 identifier-2 identifier-3

FROM BYinteger-1 integer-2

UNTIL condition-1

statement-1 …

END-PERFORM

Format

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PERFORM VARYING

• Repeatedly executes statements in loop while varying value of a field

• First identifier-1 is given FROM value

• Condition then tested

• Executes statements in loop if condition not met

• Then adds BY value to identifier-1 and repeats condition test

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PERFORM VARYING Example

Perform Varying Ctr From 1 By 1

Until Ctr > 5

Display 'Ctr = ', Ctr

End-Perform

• Sets Ctr to 1, since Ctr > 5 not true, executes DISPLAY statement

• Increments Ctr by 1, tests condition again

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PERFORM VARYING Execution

CTR Condition Output

1 1 > 5 false Ctr = 1

2 2 > 5 false Ctr = 2

3 3 > 5 false Ctr = 3

4 4 > 5 false Ctr = 4

5 5 > 5 false Ctr = 5

6 6 > 5 true (loop ends)

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PERFORM VARYING Examples

• Finds sum of odd numbers from 1 to 25

Move 0 To Total

Perform Varying Ctr From 1 By 2

Until Ctr > 25

Add Ctr To Total

End-Perform

Display 'Total = ', Total Output: Total = 169

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PERFORM VARYING Examples

• Statements to be repeated may also be in separate paragraph

Perform 300-Process-Rtn

Varying Ctr From 1 By 1

Until Ctr > 20

• Executes 300-Process-Rtn 20 Times

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Nested PERFORM VARYING

• May include a PERFORM VARYING loop as one of statements in another PERFORM VARYING loop

• Each time outer loop is repeated, inner loop is executed until its condition is met

• Following example prints the times tables for numbers 1 to 9

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Print Times Tables

Perform Varying N1 From 1 By 1

Until N1 > 9

Perform Varying N2 From 1 By 1

Until N2 > 9

Compute Product = N1 * N2

Display N, ' * ' M ' = ', Product

End-Perform

End-Perform

Outer loop

Inner loop

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Print Times Tables Execution

N1 N2 Output 1 1 1 * 1 = 1 1 2 1 * 2 = 2 ... ... ... 1 9 1 * 9 = 9 2 1 2 * 1 = 2 2 2 2 * 2 = 4 ... ... ... 2 9 2 * 9 = 18

Outer loop first time

Inner loop repeats 9 times

Outer loop second time

Inner loop repeats 9 times

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Print Times Tables Execution

• Outer loop repeated seven more times

• Each time, statements in inner loop are repeated 9 times– N2 initialized to 1 and incremented by 1

each time through inner loop until N2 > 9

• Outer loop ends after printing 9's table

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TEST AFTER Example

Perform With Test After

Until Opt-Num >=1 And <= 5

Display 'Select option (1-5)'

Accept Opt-Num

End-Perform

Example

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TEST AFTER Example

• Condition is not checked before loop begins

• DISPLAY and ACCEPT for user to enter Opt-Num always executed at least once

• Checks Opt-Num after user types in value for first time

• If Opt-Num not a value from 1 to 5, loop is repeated

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